Developmental plasticity allows individuals to invest in traits that maximize competitive success. This study found that developmental experience with social environments that vary in competitive signaling influences adult signaling behavior. In addition, through measurement of auditory catecholamines, this study elucidated potential mechanisms that could underlie the influence of the developmental environment on adult song. During development, juvenile males exposed to high-performance songs had increased amounts of dopamine metabolites in the auditory telencephalon compared to juvenile males exposed to low-performance songs. When exposed to intermediate performance songs as adults, males exposed to high-performance (compared to low-performance) songs during development had increased trill rate and trill
performance, and had lower levels of noradrenergic metabolite in their auditory telencephalon. In addition, noradrenergic activity in the auditory telencephalon was positively correlated with trill rate of the final songs that males sang prior to brain collection. These results indicate that developmental exposure to trill performance has long-term effects on adult song. In addition, these findings are consistent with the hypothesis that throughout life, catecholamines regulate changes to the brain to allow for integration of information about trill performance of the social environment.
Catecholamines in developing males
Dopamine metabolites were higher in the auditory telencephalon of males exposed to high-performance songs in development compared to males exposed to low-performance songs in development. This increase in dopamine metabolites indicates that a greater amount of dopamine was metabolized in the auditory telencephalon of males exposed to the high-
performance songs compared to males exposed to the low-performance songs. Metabolism of dopamine could occur either intracellularly or could occur after secretion from dopaminergic cells [Moore 1986; Eisenhofer et al. 2004; Meiser et al. 2013]. Importantly, dopamine itself did not significantly differ between the two treatment groups. The similarity in dopamine levels, but difference in metabolite levels suggests that dopamine was produced and then metabolized in greater amounts in the males exposed to the high-performance songs.
Dopaminergic activity in the auditory system of songbirds may mediate developmental acquisition of song or modulate song perception [Kubikova and Kostal 2010]. Dopamine is thought to encode aspects of reward [Berridge and Robinson 1998; Maney 2013] and regulate learning and synaptic plasticity [Wise 2004; Hoffmann et al. 2016], including cortical plasticity in the mammalian auditory system [Bao et al. 2001; Shepard et al. 2013]. In developing
songbirds, dopaminergic activity and receptor expression changes dramatically in the auditory system [Harding et al. 1998; Kubikova et al. 2010] in parallel to dopaminergic changes in several of the song control nuclei that regulate song learning and production [Sakaguchi and Saito 1989; Soha et al. 1996; Harding et al. 1998], consistent with the hypothesis that dopaminergic activity in the auditory system tracks aspects of vocal learning. In addition, exposure to tutor song during development increases neural activity in songbird midbrain dopaminergic cells [Nordeen et al. 2009], and pairing of auditory stimuli with electrical stimulation of midbrain dopaminergic cells
results in cortical remodeling of the auditory telencephalon in mammals, suggesting that dopamine modifies representation of auditory input [Bao et al. 2001; Shepard et al. 2013; Puschmann et al. 2014]. Therefore, the increased amounts of dopamine metabolites observed in the current study could correspond to a dopamine-dependent tuning of the auditory telencephalon during development, which could have long-term influences on song perception.
Song characteristics and catecholamines in adult males
Experience with subtle differences in the competitive environment during development changed the competitive behavior of adults. Adult males from the high-performance treatments sang songs of higher performance and higher trill rate compared to the adult males from the low- performance treatments. In addition, trill performance and trill rate of recorded songs did not differ significantly from the playback songs. These findings indicate that, like other aspects of song [Marler and Peters 1977; Marler 1997] males likely learn trill rate and trill performance from the songs they are exposed to during development [Podos et al. 2009].
Lincoln’s sparrows experience annual variation in average trill performance in the population [Sockman 2009]. This heterogeneity in the social environment indicates that in some years, males will develop and learn songs in social environments composed of high-performance songs, and in other years, males will develop and learn songs in social environments composed of low-performance songs. The results from the current study suggest that the variation in trill performance that males experience during development will affect their trill performance in adulthood, which could potentially shape the future social environment. However, if the trill performance from one year shapes the subsequent year’s social environment, I would expect trill performance in the social environment to remain stable over time, rather than vary as observed [Sockman 2009].
There are at least two potential explanations for the discrepancy between the results of the current study (that trill performance in adulthood is similar to the trill performance of the developmental social environment) and the annual variation in trill performance observed in the study population. First, there are low rates of philopatry in the study population [Sockman 2012], suggesting that males may not contribute to the trill performance of their natal social
environment. Second, previous work indicates that variation in ecological factors could contribute to variation in trill performance [Sockman 2009; Sockman 2012], and in swamp sparrows, males modulate their trill performance in competitive contexts [DuBois et al. 2009]. These studies raise the hypothesis that males are plastic in trill performance in adulthood. In a test of this hypothesis, I exposed males from both treatments to songs of intermediate
performance in adulthood. I predicted that males would modulate their trill performance over the course of exposure to the intermediate performance songs, and that the developmental treatments would influence the direction of the modulation. However, I did not detect an interactive effect of treatment and day of recording on trill performance or trill rate, indicating that the
developmental treatments did not reliably influence modulation of song. Rather, across all of the days, males from the high-performance treatments tended to sing higher performance songs than males from the low-performance treatments. While the lack of a significant interaction does not rule out the possibility that males are plastic in trill performance in adulthood, it does suggest long-term stability in the effects of the developmental social environments on adult song performance [Podos et al. 2009].
Trill rate and trill performance are performance-based traits that reflect aspects of male quality and singing capability [Podos 1997; Ballentine 2009; Byers et al. 2010; Caro et al. 2010; Vehrencamp et al. 2013; Wilson et al. 2014]. Previous studies that exposed developing swamp
sparrowsto songs with high trill rates found that, as adults, males decreased trill rate relative to the tutor songs, omitted notes from trilled syllables, or had broken syntax, in which silent gaps interspersed groups of syllables within a trill [Podos 1996; Podos et al. 1999; Podos et al. 2004; Podos et al. 2009; Lahti et al. 2011]. These studies support the hypothesis that there is a
biomechanical constraint on the ability of males to increase trill rate and performance beyond a certain limit [Podos 1996; Podos 1997; Podos et al. 2004]. A more detailed analysis would be necessary to determine if Lincoln’s sparrows from the high-performance treatments of the current study exhibited broken syntax or skipped syllables, but we did not find that males
decreased trill rate relative to their tutor songs. However, the treatment songs in the current study were within the range of trill rate and trill performance recorded from free-living Lincoln’s sparrows [Sockman 2009]. Therefore, it is likely that the high-performance songs in the current study were within the range of production capability for Lincoln’s sparrows, while that may not have been the case for the swamp sparrow studies [Podos et al. 2004; Podos et al. 2009].
Males from the high-performance treatment had lower levels of the norepinephrine metabolite MHPG in the CMM compared to males from the low-performance treatment. This finding suggests that norepinephrine secretion or intracellular metabolism was lower in the CMM of males from the high-performance group compared to the low-performance group
[Moore 1986; Eisenhofer et al. 2004; Meiser et al. 2013]. In sensory systems, including the
songbird auditory system, norepinephrine enhances responses to salient stimuli by enhancing the signal to noise ratio of neural responsiveness [Appeltants et al. 2002; Hurley et al. 2004; Cardin and Schmidt 2004; Shepard et al. 2013; Ikeda et al. 2015].
The developmental environments could have influenced adult levels of MHPG if they modulated some aspect of perception so that the intermediate-performance songs were more
salient (i.e. of relatively higher performance) for the males from the low-performance treatment compared to males from the high-performance treatment. This explanation is consistent with the patterns found in previous studies - that noradrenergic activity is greater following exposure to more salient songs [Sockman and Salvante 2008; Salvante et al. 2009; Sewall et al. 2013], and is consistent with the proposed function of norepinephrine in regulating arousal and goal directed behaviors by enhancing responses to salient stimuli [Berridge and Waterhouse 2003; Aston- Jones and Cohen 2005; Sara 2009; Shepard et al. 2013].
The modulation in perception could occur, in part, through each male’s comparison of his own songs to the intermediate performance songs [Todt and Naguib 2000; Ten Cate et al. 2002; Arnott and Elwood 2009]. Specifically, males from the low-performance treatment may have perceived the intermediate performance songs as relatively more salient, which could have increased noradrenergic release in the auditory telencephalon [Lynch et al. 2012], while males from the high-performance treatment may have perceived these same songs as relatively less salient. An alternative explanation for the levels of MHPG in the adult males is that exposure to high-performance songs in development caused a life-long decrease in noradrenergic tone. However, previous work that manipulated norepinephrine in developing songbirds suggests that norepinephrine in adulthood does not necessarily reflect levels in development [Wade et al. 2013].
Both norepinephrine and MHPG in the CMM were positively correlated with trill rate on the final day of exposure to song. This correlation is consistent with the hypothesis that
norepinephrine in CMM responds to trill rate. If the CMM is involved in mediating trill rate, it could occur through its input to HVC [Vates et al. 1996; Jarvis 2004], which is a pre-motor nucleus in the song control system [Nottebohm et al. 1982; Scharff and Nottebohm 1991; Jarvis
2004]. Previous studies in swamp sparrows found that sensorimotor cells in HVC fire in a pattern that is phase-locked to both auditory input and motor output of syllables in a song’s trill [Prather et al. 2008; Prather et al. 2009; Prather et al. 2012]. These studies indicate that HVC cells encode auditory and motor information about trill rate [Prather et al. 2008]. Therefore, noradrenergic integration of information about a song’s trill rate could, through CMM’s projections to HVC [Vates et al. 1996; Jarvis 2004], lead to modulation of trill rate.
Taken together, the results from the current study indicate that exposure to songs that differ in trill performance during development can have long-term influences on adult singing behavior [Podos et al. 2009] and potentially on perception of conspecific song. Furthermore, these results raise the hypothesis that dopaminergic activity in the developing auditory telencephalon may integrate information about the social environment as offspring develop and begin to consolidate the songs they will sing for the rest of their lives [Harding et al. 1998; Harding 2004]. During development, dopamine-induced neuroplasticity in the auditory telencephalon could modulate the representation of trill rate in the auditory system [Bao et al. 2001], such that adult song performance and the salience of the intermediate performance song (encoded by norepinephrine) differs between the high and low treatment groups. However, studies that specifically manipulate dopamine in the developing auditory telencephalon and norepinephrine in the adult auditory telencephalon are necessary to further test these hypotheses.